cxgb4: Don't sleep when mbox cmd is issued from interrupt context

When link goes down, from the interrupt handler DCB priority for the
Tx queues needs to be unset. We issue mbox command to unset the Tx queue
priority with negative timeout. In t4_wr_mbox_meat_timeout() do not sleep
when negative timeout is passed, since it is called from interrupt context.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>